Pressure sensitive transducers using a chip of a piezoresistive material, such as silicon, are now widely used. Typically, such transducers are useful in measuring and/or control systems. Such a transducer typically includes a silicon chip which has a thinned out central portion to form a thin flexible diaphragm in which are formed localized regions doped to serve as resistors whose resistance varies with the degree of flexing of the diaphragm. The resistors are then typically connected into a bridge for detecting the resistance changes and providing a measure of the pressure acting to flex the diaphragm. In such a device, for uniformity of results from chip to chip, it is important accurately to control the thickness of the diaphragm portion and the properties of the resistors. Various techniques are available for forming the diaphragm by thinning the chip but these are not completely satisfactory. Presently, the method believed most common involves forming a heavily doped surface layer in a silicon substrate, growing an epitaxial layer over such heavily doped layer and then etching away the central portion of the original substrate, using the heavily doped layer as an etch-stop for limiting the etching and assuring that the thinned region has essentially the thickness of the epitaxial layer modified by the thickness of the relatively thinner heavily doped layer. This process has a problem, since the dopants in the heavily doped layer tend to out-diffuse into the epitaxial layer as the latter is being formed. This effect, which is typically described as auto-doping, undesirably reduces the resistivity of the epitaxial layer and so affects the properties of the resistors later to be formed in such epitaxial layer.
It has also been proposed to utilize as the buried etch stop a continuous etch-resistant layer, such as of silicon nitride. However, such an etch stop normally precludes the growing of monocrystalline silicon thereover, necessitating the forming of the pressure-sensitive resistors in polycrystalline silicon. This resulted in resistors of poorer piezoresistance properties.
In the copending related application mentioned above, an apertured layer of an etch-resistant material such as silicon nitride, is buried in the silicon chip and used instead as the etch-stop, thereby to avoid the auto-doping described associated with the doped layer. Moreover, to have a monocrystalline layer in which to form the pressure-sensitive resistors, the apertures in the layer are designed to permit the layer formed thereover largely to grow epitaxially, the silicon exposed in the apertures seeding the growth, so that the grown layer is essentially monocrystalline. Moreover, the starting chip was chosen to have front and back surfaces which were cut along &lt;100&gt; crystal planes. Then to etch the substrate and to form the thinned diaphragm central portion, an anisotropic etch which preferentially etches along the &lt;100&gt; planes is used to self-limit the etching action once the etch-stop pattern is reached. This process may prove difficult to control because of its dependence on a self-limiting process based on an anisotropic etch. It is sometimes desirable to have a process free of such limitations and the present invention is so directed.
Moreover, this related application involves a process which uses a finely apertured etch resistant buried layer to form a grown layer which is essentially all monocrystalline, which sometimes proves demanding.
The present invention involves a less demanding process.